Atomization of a liquid is a process whereby the liquid is broken up and dispersed into fine droplets. Atomization is currently used in many industrial processes such as in operation of combustion engines, in liquid drying and in spray painting. One method of atomizing a liquid is accomplished by injecting a net electrostatic charge into the liquid and then passing the charged liquid through a small orifice to form a stream. Because the individual portions of the liquid each bear the same charge, small charged droplets of the liquid will form and repel from one another due to the principle of mutual repulsion of like charges. It is generally desirable in the field of electrostatic atomization to produce more finely atomized liquid droplets. To create finer droplets of liquid, the charge density of the liquid stream must be increased.
U.S. Pat. No. 4,255,777 discloses an electrostatic atomizing device which can apply substantial net charges to the liquid and which can generate fine droplets. It is possible to increase the net charge applied by the apparatus of the '777 patent so as to form finer droplets. However, when the charge on the liquid is increased to extremely high levels, the atmosphere surrounding the charged liquid may become electrically unstable and corona discharge may occur. Thus, as one increases the net charge on the stream to generate a more finely atomized liquid, the more susceptible the surrounding atmosphere becomes to corona discharge. Such corona discharge can dissipate the charge applied to the liquid, thus impeding atomization.
U.S. Pat. No. 4,605,485 discloses another electrostatic atomizing device which utilizes a blanket of gas such as sulfur hexafluoride having a high dielectric strength under pressure to surround the stream of charged liquid. This blanket of gas prevents corona discharge at relatively high charge levels.
In the apparatus of U.S. Pat. No. 4,630,169, the liquid to be charged and atomized is mixed with a high vapor pressure hydrocarbon or a halogenated component supplied through a separate line. The mixture of components is then charged and projected through the orifice. As this mixture issues as a stream through the orifice, the high vapor pressure component vaporizes and forms a gas blanket around the stream. In this apparatus as well, the gas blanket retards corona breakdown of the surrounding atmosphere.
Use of these gaseous "blankets" in the vicinity of the charged stream is helpful but limiting in that it is necessary to supply a gas or high vapor pressure component in addition to the liquid to be atomized. The extraneous gas or high vapor pressure component is objectionable in many systems.
A technique referred to as "vapor mist" insulation has been used in the unrelated art of high voltage electrical equipment. In U.S. Pat. No. 4,440,971, a sealed chamber containing high voltage electrical equipment such as a power transformer is filled with a dielectric gas supersaturated with the vapor of a dielectric liquid. The supersaturated mixture provides a high dielectric strength medium and thus retards corona discharge. In U.S. Pat. No. 4,296,003, another reference directed to high voltage electric power equipment, a sealed chamber surrounding the equipment is filled with a dielectric composition comprising a mixture of two liquids. The first liquid is selected from the group of electronegative gases (such as SF.sub.6 or F.sub.2) or the group of electropositive gases (such as N.sub.2 or CO.sub.2) or a mixture thereof. The second liquid is selected from a group of atomized liquids such as chlorinated liquids or fluorocarbon liquids or a mixture thereof. The droplets formed in such a mixture serve to enhance the dielectric strength of the gas. Neither of the above electric power references is directed to improvements in electrostatic atomization systems.
Despite efforts in the field of electrostatic atomizing devices, the promise of electrostatic atomization has not yet been fully realized due to performance limitations relating to corona discharge. Thus, there has been a long-felt need for electrostatic atomization apparatus and methods which mitigate or avoid the corona discharge problem and thus provide superior atomization of a liquid. In particular, there are needs for methods and apparatus which provide this improvement without requiring the use of additional gases or component mixtures.